The present invention relates to a golf club head, and more particularly relates to improvement in dynamic behaviour at shooting balls of the hosel of a golf club head made of metal or FRP (fiber reinforced plastics).
Conventionally, the major role expected for the hosel of a golf club head is to smoothly join the head main body to the shaft.
In the case of a wood-type club head made of persimmon, for example, its hosel is designed to smoothly converge towards the shaft for visual amenity and reduction in stress concentration. In the case wood-type club head having a metallic shell construction, its hosel is designed in the form of a straight pipe which is generally called "a pencil neck". Host conventional wood-type club heads are classified into one of the above-described two types.
In the case off a wood-type carbon club head with an FRP shell, the hosel is designed selectively in one of the two types depending on user's demand.
In use off such an wood-type club head, its shaft flexes during swing to generate repulsion which accelerates movement of the club head to pose intensive impact on a ball at the very moment of shooting.
In configuration of the main body of a club head, its face has a nominal loft angle (.alpha.) chosen in accordance with the grade of the club head. However, its actual, i.e. effective loft angle (.beta..sub.1) is dependent upon the repulsion generated by flexing of the shaft. Generally, the effective loft angle is larger than the nominal loft angle (.alpha.(.beta..sub.1). This effective loft angle (.beta..sub.1) wields a great influence on the flying course and distance of a ball shot by the club head.
As stated above, the major expected role of the hosel of a conventional club head is to smoothly join the main body to the head only and the hosel has no virtual resiliency in the ball-shooting direction. As a consequence, the effective loft angle (.beta..sub.1) at impact is dependent upon repulsion generated by flexing of the shaft only. Stated otherwise, intensity of impact is swayed by dynamic behaviour of the shaft during swing motion only.
For these reasons, when a ball is shot by a conventional golf club head, the ball is small in launch angle and large in backspin. Here, the term "launch angle" refers to the angle between the upward flying course of the ball at the very moment of impact and the horizontal direction. Whereas the term "backspin" refers to reverse rotation of the ball causing it to bounce or roll backward or stop short. As a result of small launch angle and large backspin, the ball after impact traces a low altitude course. The ball cannot fly a long distance and, after fall on the ground, cannot roll much. Thus, the ball is very poor in flying distance when shot by a conventional golf club head.
One type example of the conventional wood-type golf club head is disclosed in Japanese Patent Opening Hei. 1-94875 in which the thickness (t.sub.1) of the hosel in the ball-shooting direction (X) is larger than the thickness (t.sub.2) in the toe-to-heel direction (Y). The small thickness (t.sub.2) in the toe-to-heel direction reduces air resistance acting on the club head during swing motion. Such reduced air resistance during swing motion well avoids undesirable lowering in speed of the club head at the very moment of impact on a ball and assures appreciable increase in flying distance of the ball.
In the case of a club head with the main body made of metal or carbon, the above-described design of the hosel (t.sub.1) t.sub.2) results in smaller flexual rigidity in the toe-to-heel direction (Y) than in the ball-shooting direction (X) and, as a consequence, the hosel flexes in a normal direction (Z) with respect to the ground G.
When the hosel flexes in such a normal direction (Z), the club head undergoes so-called "toe-down deformation" at the very moment of impact on a ball. More specifically, the hosel deforms towards the ground G such that the center of gravity H of the main body falls on an extension of the axis of the shaft connected to the main body via the hosel. Due to such toe-down deformation, the ball is shot at a position of the face off the sweet spot, thereby often causing player's misstep in shot.